The many metamorphoses of the Moine

Posted on 26 December, 2016 by Metageologist

In a companion post I introduced you to a metamorphic rock with an apparently simple history. Using traditional geological techniques on this single outcrop can’t reveal the full history of the area, so this post will attempt summarise the latest research. In short1 the more closely you look, the more complicated things become.

The many Phases of the Caledonian

When I was young, things were simple. The metamorphic rocks of the Scottish Highlands, (the Moine and Dalradian) were affected the by Caledonian orogeny caused by the closing of the Iapetus Ocean. A nice simple piece of continental collision with classic Barrovian metamorphism and some splendid deformation. Nowadays, thanks to isotopic dating we know that things are much more complicated. Much much more.

From Dewey et al 2015. Our rocks is from the Moine, which is represented by the second column from the left

From Dewey et al 2015. Our rock is from the Moine, which is represented by the second column from the left

The diagram above represents the latest thinking about what the Caledonian Orogeny consisted of. Instead of a single continuous event we can recognise 3 distinct events each of which is associated with deformation and the growth of metamorphic minerals.

It comes from a paper that is a grand summary covering the entire British Isles. For today, let’s just focus on the second column from left, with Mo for Moine at the top. Start from the bottom and move up in time. First we have the Grampian event around 470 million years ago. It was caused by a collision between the edge of the Laurentian continent and an volcanic arc, with a piece of oceanic crust (an ophiolite) thrown in for good measure. I’ve written about this in detail elsewhere.

The second event, marked in the diagram as Salinian is less well understood. In the text Dewey et al. also call it the Mayoian as the deformation is well represented in County Mayo in Ireland (also perhaps because this was John Dewey’s PhD field area and he’s long been in love with the place). The alternative name Salinian (or Salinic) suggests a link with events of the same age in rocks from Newfoundland (which before the Atlantic was not far away). Dewey et al. speculate it may have been caused by subduction slab flattening (like the Laramide orogen in North America).

Bird et al. (2013) recognises the same event, dating growth of garnets at this time in rocks only about 25 km away from our rock sample. They call the event “Grampian II”, which is less poetic but avoids potentially incorrect correlations with other areas. Their proposed cause is the collision of a small fragment of crust or arc with Laurentia.

The final event is called the Scandian, which represents the final closure of the Iapetus ocean. In this northerly portion of the British Isles, this means a collision between the ancient continents of Laurentia (most of modern day North America, plus a sliver of NW Scotland) and Baltica (the ancient piece of crust that sits under modern day Scandinavia). Further north in the Moine, the dominant deformation fabrics are of this age, including the famous Moine Thrust.

Figure 5 from Bird et al. (2013) showing plate tectonic cross-sections at various times – b) is Grampian, c) Grampian II and d) is Scandian.

With one eye on the diagram above, lets describe those 3 events in terms of plate tectonics. First the Iapetus ocean opens up. Our rock is on one side, part of a plate called Laurentia. The ocean starts to close, but as it does so if forms an oceanic island arc. Around 470 Ma this collides with Laurentia heating and deforming a bunch of rocks. Twenty million years later, a smaller fragment (maybe like Rockall bank in the modern Atlantic) hits and causes some disturbance. Finally around 430 million years ago the ocean basin closes and messes some rocks up (again).

Which of these events caused the little feldspars to grow in my rocks sample? I don’t know for certain and what’s worse, it might not be any of them – it might be an older event.

When the Iapetus ocean opened at the beginning of my little story, the Moine rocks had already been deformed and heated at least twice before.

Before Iapetus

Cawood et al. (2012) is my main source of information here. Let’s cut to the chase and go straight for the summary diagram.

This is a similar diagram to the Dewey one above only it shows the actual dates from individual rocks. Similar dates exist for the younger events, but they are not shown on the first diagram.

I’ve reached my quota of explaining orogenies for the day. Let’s just say that there are two more here (Renlandian and Knoydartian), both poorly understood but both opportunities for my rock sample to have grown its feldspars.

What does it all mean?

What should we make of all this complexity? A few thoughts follow.

Firstly, the practice of correlating metamorphic or deformation events between different areas should be used with care – it can be completely wrong. The dominant fabric and metamorphic event in the northern Moine is Scandian in age, but in places further south it is Grampian II / Mayoian in age, despite looking very similar in thin section. Traditional analysis has correlated the two, but modern dating techniques show then to have formed at different times.

Secondly, if metamorphic minerals grew in the Moine at five different times, why doesn’t my rock show five sets of mineral growth? There are many possible reasons, but most importantly it’s likely that not all episodes affected all of the Moine – with the Scandian for example is possible that my area was too far from the action to be affected. Either geographically or possible because it was too high up in the crust.

It’s possible that later episodes of growth have completely destroyed earlier mineral grains, wiping the evidence away and making the rock’s history look simple. But we only know about each episode because we’ve dated a mineral grain that has remained relatively unscathed since. Rocks nearby still have visible sedimentary features retained so there are limits to how much we can explain this way.

Some believe that metamorphic events may transform relatively small volumes of rock because heating or fluid flow is localised. Outside of these areas, dry rocks are heated but don’t recrystallise – don’t change. A piece of pottery is a form of metamorphic rock transformed from mud, but heating it up again doesn’t cause a further dramatic change. Maybe my rock was transformed early on when it was rich in water from metamorphic reactions, but when heated up at a later date, little changed.

A third topic is around structure. There are no complex structures in my rock sample and its outcrop, but the Moine as a whole is a classic locality for refolded folds, which are exactly what you’d expect in an area deformed multiple times. Individual areas may have up to 4 different phases of folding, but the relationship between these and each orogeny is not simple. Bird et al. discuss this topic (about another area) and suggest fabrics or folds may be composite, that “structures may have initially developed as tight to open structures during the Late Ordovician event, and were later strongly modified into their present tight to isoclinal, sheath-like geometry during intense shear associated with Scandian nappe stacking“.

Many of discussions around metamorphism are relevant here as well. Let’s remember the undeformed sedimentary features in rocks nearby. They may now be vertical, but they are otherwise little deformed.

So how old are these blasted feldspars anyway?

Krabbendam et al. (2014), studying Moine rocks not far north of mine regard early structures (D1) as being pre-Grampian and later ones – and the main phase of mineral growth – as being Grampian II in age.

So that’s the most likely answer for my rock. Whatever happened in the pre-Grampian events is lost in the mists of time. Those nice big white feldspars grew and were rotated during the Grampian II event and shrugged off the Scandian event without any visible changes.

If you are thinking I’ve made this unnecessarily complicated2 I’ll just point that on a hillside visible from my outcrop there are some of the rocks that were the basement on which the Moine sediments were deposited. These ‘Lewisianoid’3 rocks, now ‘inliers’, or strips of gneiss folded into the Moine metamorphic rocks, have experienced all of the events the Moine has, plus three or more others (Scourian, Laxfordian and Grenvillian4). Here’s a thin section of some, with folded banding, but not looking *that* complicated, considering the 8 different events it’s witnessed.

Life is complicated. Plate tectonics is a continuous process that takes place in three dimensions on a sphere over millions of years. The fact we can use a bunch of ancient rocks in Scotland to recreate the ancient dance of vanished oceans and transformed continents is a triumph of science, but let’s not be surprised that it is complicated.

REFERENCES

Note that Professor Rob Strachan of ~~Plymouth~~ Portsmouth University is a co-author on nearly all of these papers. His nickname of ‘Captain Caledonides’ is well-earned.

Dewey, J. F., Dalziel, I. W., Reavy, R. J., & Strachan, R. A. (2015). The Neoproterozoic to Mid-Devonian evolution of Scotland: a review and unresolved issues. Scottish Journal of Geology, 51(1), 5-30. doi: 10.1144/sjg2014-007

Bird, A. F., Thirlwall, M. F., Strachan, R. A., & Manning, C. J. (2013). Lu–Hf and Sm–Nd dating of metamorphic garnet: evidence for multiple accretion events during the Caledonian orogeny in Scotland. Journal of the Geological Society, 170(2), 301-317. doi:10.1144/jgs2012-083

Cawood, P. A., Strachan, R. A., Merle, R. E., Millar, I. L., Loewy, S. L., Dalziel, I. W., … & Connelly, J. N. (2015). Neoproterozoic to early Paleozoic extensional and compressional history of East Laurentian margin sequences: The Moine Supergroup, Scottish Caledonides. Geological Society of America Bulletin, 127(3-4), 349-371. doi: 10.1130/B31068.1;

Krabbendam, M., Leslie, A. G., & Goodenough, K. M. (2014). Structure and stratigraphy of the Morar Group in Knoydart, NW Highlands: implications for the history of the Moine Nappe and stratigraphic links between the Moine and Torridonian successions. Scottish Journal of Geology, 50(2), 125-142. doi: 10.1144/sjg2014-002

All diagrams reproduced under fair use policy.

The deceptive simplicity of a metamorphic rock

Posted on 19 December, 2016 by Metageologist

I’d like to introduce you to a rock.

Pretty isn’t it? The white crystals caught my eye, as they did that of three different geologists of the British Geological Survey, who between them collected 5 different samples from the same small area of Scotland.

When did these crystals grow? How old are they? These rocks here are part of the Moine supergroup which started as a pile of sediments a billion years ago and the last geological event in this part of Scotland was a mere 60 million years ago, so there’s a wide possible range.

The first and easiest tool available to a geologist is to establish the age of something relative to other events. The white spots are potassium feldspar that grew when the rock was metamorphosed – changed from a muddy sand into something (even) more interesting. Metamorphosis is most often associated with geological structures. Minerals most often form because rocks are buried deep and heated and this squashes them, flattening or folding the sedimentary layers and metamorphic minerals alike.

Yes the sea is azure blue and the beach empty. It was a fabulous week’s holiday.

In this picture we see a set of lines folded round which are original sedimentary layers. Our rock comes from the darker layer to the right. The white spots within it are roughly flattened along a plane that passes through the middle of the fold. This suggests they formed at the same time as the rocks were folded.

There is a little more detail to be seen in thin sections made by the state-funded geologists who have passed here before me1.

http://www.bgs.ac.uk/data/britrocks/britrocks.cfc?method=viewSamples&sampleId=351198

Image taken from BGS thin section image archive

Here we are looking down a microscope at the light that has passed through a thin slice of the rock – we are peering into its soul. The plain white areas are the feldspar crystals which we can call porphyroblasts if we are feeling fancy (meaning they grew as big crystals during metamorphism). Notice also the patterns made by the long and thin red-brown and grey mica crystals. There are little folds.

With more magnification (and in cross polarised light, so the minerals look different) you can see that the feldspars contain grains of other minerals that a strung out in lines. These are minerals that were swallowed by the growing feldspar and give a glimpse of earlier structures.

Here’s one interpretation of what is going on. An early alignment of the minerals was parallel to the sedimentary bedding. This was horizontal in our field of view. The feldspar grains grew over this fabric. Later the rock was squashed in a different direction, causing the folding we see in the outcrop and in the thin section. The mica grains are now mostly vertical with only a few areas staying flat. Some feldspar grains stayed put, but most have been rotated so that their long axes are vertical.

I’ve deliberately gone for the most simple explanation, but it’s a plausible one based on what we see in the outcrop. Two sets of squashing and one phase of mineral growth. Nevertheless its likely that we are not seeing the full picture here. The same package of rocks looks very different depending on where you are. About five kilometres west of where my rock came from, similar Moine sediments have vertical layers, but so little deformed that sedimentary cross bedding is visible.

Cross bedding in vertical Moine sediments

Nearby there are folds, but these were formed when it was still sediment, the layers folding due to slipping of sand. You can tell this because the layers either side of the folds are flat.

Soft sediment deformation in vertically bedded Moine sediments

Go ten kilometres east and the you are still in Moine sediments, but they are rather more intensely deformed and metamorphosed. Here the original sedimentary layers are stretched out into layers as thin as centimetre.

Intense folding in Moine sediments, near the Sgurr Beag thrust.

Clearly, it’s important not just to look at a single outcrop – which is where geological mapping comes in. This shows that these metamorphic rocks are part of a wide area over northern Scotland. This is unconformably overlain by undeformed sediments of Devonian age. So sometime between 1000 and 416 million years ago these sediments were heated and folded – that’s when the white crystals grew.

These are old techniques and technology marches on. Modern earth scientists, armed with sophisticated machines, scary acid and an understanding of radioactive decay are able to date the age of metamorphic events and even directly date the age of individual metamorphic minerals.

The Moine rocks of Scotland are well studied. Bring together hundreds of radiometric dates, highly detailed mapping and the study of thousands of outcrops and thin sections and you get a picture of almost terrifying complexity.

It turns out that the white grains in my rock sample with its apparently simple history could have formed in any one of at least five different occasions when metamorphic minerals formed in the area. Each one of these represents a significant event – an ocean closing, an arc smashing itself into oblivion against an unyielding continent – yet somehow a single rock shows only a single part of this saga.

I’ll tell this complicated geological history, and why it’s not visible in a single outcrop in another post.

Stirring tales from the deep past.

Posted on 12 April, 2016 by Metageologist

My cup of tea is sitting nearby1, the rocket-fuel for the mind is sitting in a piece of man-made metamorphic rock and lying on the saucer is a humble object that bears mute witness to ancient, earth-changing events.

Tea in England is typically taken with milk and sometimes with sugar – lots if it’s “builders’ tea” – and a small spoon is required to add and blend the ingredients. These spoons can be made of silver or even gold. I’m not a Duke or a Prince, so mine are made of stainless steel.

Steel is an impure form of iron and has been made for thousands of years. The use of Archaeological periods (Iron Age succeeding Bronze Age) works because smelting iron tools is harder to do, but gives a better product than bronze. Strong and sharp iron tools are excellent for slicing through both fields and people. Successful Iron Age societies such as the Roman Empire were based on both swords and ploughshares, working together.

Iron-carbon phase diagram. From Wikipedia

Much as with ceramics, small differences in the processes used can make a huge difference in quality of the end product. Molten iron easily mixes with carbon, which was originally introduced from the burning charcoal used in the furnaces. Mixing different amounts of carbon within iron results in different phases (with different atomic structures) – as the phase diagram above shows. This sort of diagram will be familiar to anyone who’s studied igneous petrology. It can be used to predict which minerals are produced in which order when a molten substance is cooled. Producing a molten mix of carbon and iron of the correct composition and cooling it at a rapid rate results in a fine-grained mix of different materials that is strong yet not brittle.

Our ancestors worked all the best forms of steel by trial and error over generations and in many different places. Getting hold of iron ore was never a major issue as iron is one of the most common elements on earth and beyond. It’s atomic nucleus is one of the most stable and a common product of stellar furnaces. Indeed the earth and other rocky planets are so rich in iron that as well as forming part of many silicate minerals in the mantle and crust, left over iron trickled down into the centre of the earth where it forms a dense magnetic core.

Banded Iron Formation. Source: James St. John on Flickr

Much modern iron comes from deposits that have their roots in a remarkable transformation of the earth- the “great oxidation event”. The early earth was a very different place with no oxygen roaming ‘free’ in the air – this reactive element was everywhere bound up with other elements. Iron at the surface was mostly in the reduced ferrous form of iron that easily forms compounds soluble in water. The first forms of life that lived via photosynthesis were tremendously disruptive. The Oxygen they produced never reached the atmosphere but quickly reacted with the surrounding seawater. Often it reacted with iron, changing it from ferrous into ferric iron that forms compounds that are not soluble in water. These same compounds are familiar to us as red rust.

Slowly, photosynthesising organisms turned dissolved iron into layers of iron minerals on the sea-bed. Over several billions of years, grain by grain, these bacteria produced huge volumes of iron-rich sediment. These banded iron formations are found in ancient rocks around the world and form the great iron-ore mines of Western Australia that have helped build modern industrial China.
Eventually the critters won. The earth ran out of ferric iron and the oxygen stopped reacting in the sea and started bubbling into the atmosphere, building a world, our world, that would in time support oxygen breathing animals who sip tea.

The iron in my spoon, was once transformed by the caress of ancient slime, but now it is nearly pure metal, strong and shiny. Surrounded by Oxygen, it is under threat – the Oxygen could bind with it again, undoing the smelting process and turning it back into rust. To counter this – to make it stainless steel – just over 10% of Chromium has been blended in. Chromium is not resistant to Oxygen’s charms either, but the resulting oxide does not let Oxygen pass through. This process of passivation means my spoon has a thin protective film over it, keeping it shiny even in the hostile toxic environment of my tea cup.

Chromite in Serpentinite. Source: James St. John on Flickr

The Chromium in my spoon probably came from South Africa. It is a relatively uncommon element and places where it is concentrated enough to mine are not easy to find. The best place is in a large igneous intrusion formed from the melting of the earth’s mantle. The magma itself isn’t very rich in Chromium, but the biggest intrusions cool very slowly, which allows layers of different minerals to form. We’re still not exactly sure what the exact processes are, whether the minerals sink to the bottom of a pool of molten rock or form later at the slushy mushy semi-solid stage (probably it’s a bit of both). As the owner of a spoon we don’t need to know, just to be grateful that some of the layers are so rich in a mineral called Chromite they form a rock called chromitite.

There is a massive frozen magma chamber where this has happened in South Africa. Called the Bushveld intrusion it is rich in Chromite, and also most of the earth’s known reserves of Platinum group elements. It was formed from a million km³ of magma that was intruded into the crust 2 billion years ago. To put that in context, that would fill the Baltic Sea fives times over. Four Bushveld’s worth of magma would fill the Mediterranean Sea with some left to slop over the side, making a hell of a mess. The Bushveld is an amazing thing, if a little overshadowed by the gold and diamond mines and the massive eroded meteorite crater that sit nearby.

Science and history isn’t just something that sits out there somewhere, in museums, or distant galaxies. It can be found within the most ordinary of objects. These stories aren’t just stories. We can’t identify which ones, but some of the iron atoms in my teaspoon were affected by the great oxidation event. The same atoms really were there billions of years ago. My choice of tea as a drink is not a simple act of will; I’m guided to it by global events hundreds of years ago. Look deeply into anything around you and there are amazing stories to tell.

Man-made metamorphic rocks

Posted on 30 March, 2016 by Metageologist

There’s a cup of tea next to me, steaming gently. I’ve already written about the history of the drink, how a Chinese herb ended up defining Englishness and having the power to create riots in Ireland. But what of the cup? It’s a posh one – not a thick heavy earthenware mug but a slightly translucent piece of porcelain, strong enough to be made into thin delicate shapes. Like the tea within it’s on my desk thanks to early modern globalisation but it is also a type of metamorphic rock, of anthropogenic facies1 as you shall see.

Source

The cup comes from my granny’s ‘China tea set’. In England, fine elegant pieces of porcelain are associated with China, as that’s where sometime between the 7th and 9th Centuries porcelain was invented. It soon made it’s way along the Silk Road through Central Asia to the sophisticated cities of the Islamic world2. Poor and backward Europe didn’t see much porcelain until we worked out how to bypass the Silk Road by ship.

The ships that started the trade in tea with China in the 16th Century, sailed East laden with silver bullion as this was the only thing the Chinese wanted from the West. The silver itself was mostly mined from the South and Central American mines run by the Spanish, but that’s another story.

Ships that swapped silver for tea and silks had a problem. These Eastern luxuries are light and the ships were designed to sail best when sitting low in the water. Needing to add weight – ballast – they shipped back something heavy that they could sell in Europe – Chinese porcelain. Starting with royalty a taste for ‘fine China’ spread across Europe from the 16th Century. In a pleasing symmetry the finest examples were, in Europe, as valuable as silver. This beautiful strong translucent material was used to make plates, bowls, cups, tulip holders….

Porcelain is not the same as ‘earthenware’ or ‘stoneware’ that Europeans were already familiar with. Take most forms of clay and bake them at high temperatures and they form some sort of pottery. Clay minerals have a platy layered structure dominated by a lattice of aluminium and silicon oxides. Water is always to found between these sheets3 but also a wide variety of other elements. Clay minerals form during weathering of other rocks – where water and time break up tidier mineral structures that formed deep in the earth but are less suited to existing at the surface.

When heated, clay minerals break down. The water hidden inside them becomes keen to escape, driving chemical reactions that build new minerals. When a clay-mineral rich rock is buried in the guts of a mountain range, it is transformed into a metamorphic rock – a schist say – where aluminium rich metamorphic minerals grow – micas, Garnet, Staurolite and many more. If you’re really lucky the rock will partially melt, resulting in an migmatite. Taking clay and baking it in a kiln is the same process – a human created (anthropogenic) form of metamorphism.

Human metamorphism is much quicker than the natural form and at much lower pressure, but occurs at a much higher temperature (>1200°C for porcelain). The minerals and textures that form are therefore different, notably the grain size is much smaller. There is also partial melting of some minerals in the clay, which helps to bind the material together. In standard pottery, the clay contains a mixture of different clay minerals so a variety of new minerals form, giving it a generic brown sort of colour.

Kaolin

When Europeans first encountered porcelain, it was like nothing they’d ever seen. Strong, fine and translucent, with a pure white colour. Almost immediately they attempted to copy it. Early attempts involving ground glass and ash from bones. This soft-paste porcelain had the desire translucent quality but was softer. Only in the 18th Century did Europeans manage to produce the real thing, first in Dresden in Germany and then in France (Sèvres) and Britain.

The secret was to start with the right ingredients – nearly pure quantities of a particular clay mineral called Kaolinite (found in kaolin, or “china clay”), mixed with a common mineral called feldspar. Kaolinite – Al₂Si₂O₅(OH)₄– forms from the breakdown of feldspar under the action of hot water. For all three of the early European sites of porcelain manufacture, the kaolin came from granitic rocks4. The English deposits in Cornwall are still being mined- here soon after it formed, the cooling granite pulled in groundwater, heated it and pumped it around, rotting itself from the inside.

China Clay pits visible from Space in Cornwall. Image from Wikipedia

Just like the progression from mudrock to slate to schist to gneiss, the metamorphic process to form porcelain has various stages5. First the water is driven off leaving a disordered material called metakaolin (Al₂Si₂O₇). Between 900 °C and 1000 °C a new mineral phase forms, with a spinel structure. Above 1050 °C the mineral Mullite (Al₆Si₂O₁₃) forms. This mineral was first found in nature on the Scottish island of Mull6 where small quantities of muddy rock were engulfed in lava and so heated above 1000 °C – nature’s own failed attempt to make porcelain. Within baking porcelain, Mullite initially forms in the shape of plates (platelet Mullite) but above 1400 °C the minerals start forming in the shape of needles. Plates can slide against each other, but needles cross and interlock with other, so this final change to the shape gives porcelain its great strength.

These are tremendous temperatures – it’s rare for rocks to experience such temperatures (except for within the deep earth) and Mullite is unusual in thriving under such conditions. Hessian crucibles were containers prized by alchemists and early chemists across Europe for their ability to survive whatever flames and chemicals were inflicted on them. Made to a secret recipe, only recently was it discovered that they are rich in Mullite.

Alchemists were in the business of finding miraculous transformations. The person credited with first discovering the secret of porcelain in Europe, Johann Friedrich Böttger, the “porcelain prisoner” was an alchemist imprisoned and instructed to turn lead into gold. After 14 years, having discovered a different form of profitable transmutation and brought the Meissen porcelain factory into being, he was finally released.

What to our ancestors seemed miraculous we take for granted. Let’s take a sip and pause to thank those people – Chinese and European – whose hard work lets us transmute mud into practical elegance.